Composite injection mouldable material

ABSTRACT

An injection mouldable material and a method of making the material wherein the material comprises a mixture of an injection-mouldable polymer and a dielectric substance having a relatively high dielectric constant. The dielectric substance is preferably a ceramic. Such material is particularly suitable for the manufacture of planar antennas. The polymer preferably has a loss tangent equal to or less than 0.01 and greater than or equal to 0.002.

CROSS-REFERENCE TO RELATED APPLICATION

This is a 371 of international application PCT/EP99/03714, with aninternational filing date of May 28, 1999.

BACKGROUND OF THE INVENTION

The present invention relates to composite injection mouldable materialand in particular to platable injection mouldable material having arelatively high dielectric constant and low dielectric loss.

Applications such as RF (Radio Frequency) applications require deviceshaving a high dielectric constant and a low dielectric loss. It is knownto use metallised ceramic components for these applications, producedusing established technology to cast and fire the ceramic. However, themachining of these components is generally difficult and only simpleplanar structures are possible. Metallisation to form interconnectionand tracking is also a specialist activity and the resulting componentstend to be heavy.

Composites of polytetrafluoroethylene (PTFE) and ceramic have foundincreasing use in similar applications. The composite is more easilymachined and patterned than ceramics. However, it is may only be made insheet form.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention an injectionmouldable material comprises a composite of an injection-mouldablepolymer and a ceramic filler.

Preferably the loss tangent of the polymer is equal or less than 0.010(which corresponds to a Q of 100) and greater than or equal to 0.002(which corresponds to a Q of 500) at 1 GHz. Preferably thesecharacteristics are maintained for frequencies up to 2.5 GHz.

In accordance with a second aspect of the invention an injectionmouldable material comprises a composite of an injection-mouldablepolymer and a filler having a dielectric constant of at least 9.

Thus the invention enables the production of devices requiring a highdielectric constant and low loss, which are light and capable of beinginjection moulded. Complex 3D structures may be constructed which may beplated, preferably using Moulded Interconnection Device (MID)technology.

The filler is chosen to have a high dielectric constant compared withthe polymer and low dielectric loss. A dielectric constant in the rangeof 9-250 is contemplated for the filler.

The material has a relatively high dielectric constant which allows areduction in the physical size of RF devices such as antennas. This hasparticular application to internal antennas where market forces aredemanding ever smaller devices. In addition a material having a highdielectric constant allows better control of field patterns, thus makingdevices more directional and efficient. The low dissipation factors ofthe material also minimise the energy loss in the device structure andhence maximise the energy radiated.

Preferably the polymer is polyetherimide and the filler is a ceramic.Another particularly suitable polymer is SPS (synthiotacticpolystyrene). These polymers are chosen for their good plate-abilitycharacteristics.

The filler may have a dielectric constant of at least 9.8, preferably atleast 30 or 100. Preferably the filler is Titanium, Ba—Ti or Alumina.

Advantageously the filler may comprise at least 10% by weight of thematerial and preferably at least 30% to 60% by weight of the material.Clearly the higher the percentage of the filler, the higher thedielectric constant of the composite material will be. The amount offiller which may be used is generally limited by the mechanicalproperties of the resulting material e.g. polyetherimide with more than60% Titania may become too brittle or not suitable for injectionmoulding.

Devices formed of a material according to the invention are thus lighterthan prior known devices formed from ceramic and may be formed intocomplex structures. The material is particularly suitable for themanufacture of antennas although it is suitable for many other highfrequency applications.

In a further aspect of the invention, a method of manufacturing acomposite material comprises mixing a ceramic substance with aninjection-mouldable polymer, heating the mixture and extruding thecomposite material.

Preferably the ceramic substance is in a powdered form and the polymeris in a granulated form.

In a yet further aspect of the invention, a method of manufacturing adevice comprises injection moulding material comprising a composite ofan injection-mouldable polymer and a ceramic substance to form thedevice.

In a yet further aspect of the invention there is provided an antennaformed from a material comprising an injection mouldable polymer and aceramic substance. Such an antenna is particularly suitable for use withportable communication devices, for example radio telephones, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 shows the dielectric constant of one embodiment of the materialfor differing percentages of Titania;

FIG. 2 shows an example of production apparatus for producing materialaccording the invention;

FIG. 3 shows an example of injection moulding apparatus for use withmaterial according to the invention;

FIG. 4 shows an example of a planar antenna formed of material accordingto the invention;

FIG. 5 shows an example of an antenna having a conductive filamentdisposed on a support formed of material according to the invention;

FIG. 6 shows an example of a helical antenna having a core formed ofmaterial according to the invention;

FIG. 7 shows another example of an injection moulded planar antenna;

FIG. 8 shows an example of an injection moulded antenna having acapacitive load; and

FIG. 9 shows an example of an injection moulded antenna having aprotective shield at one edge of the antenna.

DETAILED DESCRIPTION OF THE INVENTION

The material comprises a composite of an injection-mouldable polymer anda filler having a dielectric constant which is relatively high comparedwith that of the polymer. One embodiment of the material comprises acomposite of polyetherimide and a ceramic. Polyetherimide is availablefrom GE Plastics in the Netherlands under the brand name Ultem.Polyetherimide was chosen since it is an amorphous material which isreadily platable. It has good dimensional stability (low creep andcoefficient of thermal expansion), chemical resistance suitable forplating and is stable to high temperatures. In addition polyetherimidehas a relatively high dielectric constant (2.9) and a low loss tangent(0.0025 at 1 GHz) for a polymer. Other injection mouldable polymers maybe used.

The filler has a dielectric constant which is relatively high comparedwith that of the polymer. Table 1 shows the dielectric constant and losstangent for four ceramics.

TABLE 1 Ceramic Chemical Name Dielectric constant Loss tangent DA-9Alumina 9.8 0.0001 D-38 Ba—Ti 37.0 0.0005 D-8800 Ba—Ti 38.6 0.0002 D-100Titania 100.0 0.0010

These ceramics are available from Trans-tech Inc of the USA. The ceramicis in the form of fully fired spherical powder having a 325 mesh(0.044mm) particle size.

Table 2 shows the resulting dielectric constant of the compositematerial for given loadings of the polymer with each of the fourceramics of Table 1.

TABLE 2 dielectric constant By Weight Filler 10% 15% 25% 30% 35% 60%None 2.7 DA-9 3.20 3.50 3.76 4.38 D-38 3.22 3.89 4.05 6.93 D-8800 3.304.03 7.44 D-100 3.23 3.66 4.81 10.21

As can be seen from Table 2, when the ceramic makes up only 10% of thematerial, the dielectric constant of the composite material issignificantly improved compared with the unloaded polymer. Clearlyhigher dielectric constants are achieved with higher percentages of thefiller.

Table 3 shows the resulting dielectric loss of the composite materialfor a given loading of the material with each of the four ceramics ofTable 1.

TABLE 3 loss tangent By Weight Filler 10% 30% 60% None 0.0025 DA-90.0001 0.0007 D-38 0.0001 0.0008 0.0007 D-8800 0.0012 0.0008 0.0008D-100 0.0007 0.0015 0.0030

Although loss figures are difficult to measure and may be inaccurate,overall the figures show a reduction in loss compared with the polymeritself. Testing at 1 MHz and 1 GHz gave similar results for both thedielectric constant and the loss.

The preferred composite tested was polyetherimide with 60% b.w Titania(D-100). FIG. 1 shows the expected dielectric constant of the compositematerial for different percentages (by weight) of Titania.

Plating tests were carried out on the resulting composite materials.Using standard plating processes, all composite materials platedsuccessfully with varying levels of adhesion and surface appearance. Thehigher percentage filled materials showed the best adhesion.

Table 4 shows the density of the composite materials in g/cm³. Thedensity of the ceramics themselves ranged between 4.0 and 4.7.

TABLE 4 density By weight Filler 10% 15% 25% 30% 35% 60% Ultem 1.27 DA-91.39 1.63 1.70 2.18 D-38 1.40 1.58 1.65 2.27 D-8800 1.40 1.66 2.30 D-1001.39 1.45 1.63 2.18

The material may be made using conventional kneading and extrudingprocesses as shown in FIG. 2. The polymer is introduced into a firstkneading chamber 20 which breaks down the granular polymer into smallerparticles. These are then passed into a second chamber 22 which heatsand kneads the polymer into even smaller particles. The ceramic powderis then added into a third chamber 24 and the composite material ispassed through three more chambers 25, 26, 27 where it is heated andkneaded to form an evenly distributed composite material which is thenextruded as a bar of material through outlet 28. The bar is then cooled.The composite material output from the apparatus of FIG. 2 may beprocessed further e.g. formed into portions of a size suitable for theirintended use.

Other materials may be added, for instance into chamber 25, to impartother desired mechanical properties to the material.

FIG. 3 shows an example of the injection moulding process. In this case,the composite material has been formed into chips. The composite isintroduced via inlet 30 into an injection nozzle 32. The powder isheated by heating element 33 and transferred to a mould 34 underpressure supplied by a piston 35. The soft material in the cavity 36 ofthe mould cools rapidly and can be quickly ejected.

The material is suitable for any device requiring a high dielectricconstant and low dielectric loss. It is particularly suitable for use inthe manufacture of antennas, the high dielectric constant providing acloser near field. By increasing the dielectric constant of thematerial, which results in a reduced electrical length, smaller devicescan be made.

The material has many potential applications. For example the materialis particularly suitable for planar antennas (for instance as used inmobile portable telephones); 2D and 3D microwave and RF circuit boards;multichip technology; RF and microwave cables and couplings; loop,satellite and GPS antennas; and base station antennas. FIG. 4 shows anexample of an injection moulded planar antenna 40 formed of materialaccording to the invention.

The material may be used to form a supporting core for a helical antennaor for a flat linear antenna. FIG. 5 shows an example of an antennahaving a conductive filament 50 disposed on a support 52 formed ofmaterial according to the invention. FIG. 6 shows an example of ahelical antenna 60 having a core 62 formed of material according to theinvention. Further examples of such antennas may be found in UK patentno. 1367232 and European patent application no. 0198578.

Owing to the injection-mouldable nature of the material, complex 3Dstructures can be produced e.g. to act as internal antennas. FIG. 7shows an example of a planar antenna 70 (e.g. of the so-called PIFtype). The antenna 70 comprises a moulded structure 72 which includesair pockets 74. The combination of air 74 and material 72 allows adesigner to change the effective dielectric constant of the antenna.Thus antennas may be of the same size but have different effectivedielectric constants. This is particularly attractive to devicemanufacturers where a single casing may be used to house devices ofdiffering capabilities.

FIG. 8 shows another example of an antenna in which the planar antenna80 is moulded to have a capacitive load 82 at one end. This allows theresonant frequency of the antenna to be tuned.

The antenna structure may be moulded with a concentration of thematerial in a particular place. For instance, FIG. 9 shows an antenna 90having a skirt 92 of the material around one end of the antenna tocontrol and direct the radiation field of the antenna. In the embodimentshown in FIG. 9, the radiation field is concentrated around the antennaand hence the amount of radiation from the antenna directed towardscomponents to the left of the antenna as viewed in FIG. 9 is reduced.

By moulding the antenna and its structure from a high dielectricmaterial it is possible to design the shape of the antenna to affect andcontrol the radiation field patterns. This can be used to reduce thesize of the antenna and the handset by reducing the effects of nearbycomponents and absorptive structures. The high dielectric materialincreases the effective space between the antenna and nearby components,thereby allowing them to be brought closer together without detrimentaleffect.

The material may be used to form microwave or RF circuit boards. Thematerial may also be used to mould RF and microwave cables and couplingseither separately from or directly attached to circuit boards orenclosures. The material may also be used to form reduced-volumeantennas for satellite, base station and GPS antennas.

The material may also be used to form devices such as dielectricresonators, filters etc.

Generally the material will find applications in most high frequencyapplications. The above examples are not intended to limit theapplications for which the material of the invention is suitable.

What is claimed is:
 1. An injection mouldable material suitable forforming an antenna, said injection mouldable material comprising: acomposite of a platable, injection-mouldable polymer and a ceramicsubstance.
 2. An injection mouldable material according to claim 1wherein the polymer has a loss tangent equal or less than 0.01.
 3. Aninjection mouldable material according to claim 1 wherein the polymerhas a loss tangent equal or greater than 0.002.
 4. An injectionmouldable material according to claim 1 wherein the polymer has a losstangent substantially equal to 0.0025.
 5. An injection mouldablematerial according to claim 1 wherein the polymer is polyetherimide. 6.An injection mouldable material according to claim 1 wherein the ceramichas a dielectric constant of at least 9.8.
 7. An injection mouldablematerial according to claim 1 wherein the ceramic has a dielectricconstant of at least 9.8.
 8. An injection mouldable material accordingto claim 1 wherein the ceramic has a dielectric constant of at least 30.9. An injection mouldable material according to claim 1 wherein theceramic has a dielectric constant of at least
 100. 10. An injectionmouldable material according to claim 1 wherein the ceramic comprises atleast 10% by weight of the material.
 11. An injection mouldable materialaccording to claim 1 wherein the ceramic comprises at least 30% byweight of the material.
 12. An injection mouldable material according toclaim 1 wherein the ceramic comprises at least 60% by weight of thematerial.
 13. An injection mouldable material according to claim 2,wherein the polymer has a loss tangent equal or greater than 0.002. 14.An injection mouldable material suitable for forming an antenna, saidinjection mouldable material comprising: a composite of a platable,injection-mouldable polymer and a substance having a dielectric constantof at least
 9. 15. An injection mouldable material according to claim 14wherein the substance is a ceramic.
 16. An antenna comprising: aninjection mouldable material suitable for forming an antenna, saidinjection mouldable material comprises: a composite of a platable,injection-moulded polymer and a ceramic substance.
 17. An antennaaccording to claim 16 wherein said antenna is of the planar type.
 18. Anantenna comprising: a core included within said antenna of an injectionmouldable material suitable for forming an antenna, said injectionmouldable material comprises: a composite of a platable,injection-mouldable polymer and a ceramic substance.
 19. An antennacomprising: a conductive filament disposed of a support, wherein thesupport is formed of an injection mouldable material suitable forforming an antenna, said injection mouldable material comprises: acomposite of a platable, injection-mouldable polymer and a ceramicsubstance.
 20. A method of manufacturing an antenna, said methodcomprising: an injection moulding material comprising a composite of aplatable, injection-mouldable polymer having a loss tangent equal orless than 0.01 and greater than or equal to 0.002 and a ceramicsubstance to form the device.
 21. An injection moulded antennacomprising: a planar sheet of injection mouldable material, saidinjection mouldable material comprises: a composite of a platable,injection-mouldable polymer and a ceramic substance, and a skirt at atleast one edge of the sheet to prevent radiation from leaking in thedirection of the skirt.
 22. An injection mouldable material suitable forforming an antenna, said injection mouldable material comprising: acomposite of a platable, injection-mouldable polymer and a ceramicsubstance, wherein the ceramic substance is a ceramic powder.
 23. Theinjection mouldable material of claim 22, wherein the platable,injection-mouldable polymer is polyetherimide.
 24. The injectionmouldable material of claim 22 wherein the material has a dielectricconstant of at least 3.20.
 25. The injection mouldable material of claim22 wherein the material has a dielectric constant of at least 3.50. 26.The injection mouldable material of claim 22 wherein the material has adielectric constant of at least 4.48.